1. Field of the Invention
The present invention relates to a driving device for driving a lens unit constituting an imaging optical system of an image sensing apparatus such as a camera phone for zoom control or focus control, as well as a driving mechanism, and an image sensing apparatus incorporated with the driving device.
2. Description of the Background Art
In recent years, a high-quality camera phone or a like device has been developed. For instance, the number of pixels of an image sensor to be loaded in a camera phone has been spectacularly increased. As the high-quality camera phone has been developed, a high-performance lens unit constituting an imaging optical system has also been demanded. Specifically, a fixed focal length system is replaced by an auto-focus system. Concerning a zoom function, an optical zoom function has been demanded in place of or in addition to a digital zoom function. A lens driving mechanism for moving a lens element in an optical axis direction is necessary both in the auto-focus system and the optical zoom system.
Heretofore, a moving coil system has been widely used in the lens driving mechanism. The lens driving mechanism using the moving coil system is constructed by mounting a pair of a coil and a magnet onto a lens frame member for fixedly holding a lens element and a fixed frame member for holding the lens frame member, respectively, to drive the lens frame member in an optical axis direction, utilizing a magnetic repulsion force depending on an energization amount to the coil. In the lens driving mechanism using the moving coil system, use of the coil and the magnet, whose miniaturization is difficult, is indispensable. This does not follow a recent trend of seeking for reduction of a device in size and weight. Also, it is necessary to mount the coil of a relatively heavy weight on the lens frame member. This may increase an inertia moment of the lens frame member when an impact is applied thereto, thereby lowering the impact resistance.
A lens driving mechanism using a shape memory alloy (hereinafter, also called as “SMA”) actuator is known in place of the lens driving mechanism using the moving coil system. In use of the lens driving mechanism with the SMA actuator, a contraction force is generated by energizing and heating the SMA actuator, and the contraction force is used as a lens driving force. Generally, it is easy to reduce the size and the weight of the lens driving mechanism with the SMA actuator. Further, the lens driving mechanism with the SMA actuator is advantageous in obtaining a relatively large magnitude of force.
Several patent documents (D1 through D4) disclose known structures concerning a lens driving mechanism with an SMA actuator. Japanese Unexamined Patent Publication No. 9-127398 (D1) discloses an arrangement, wherein an SMA spring and a bias spring are arranged resiliently against each other with respect to a moving lens element. Japanese Unexamined Patent Publication No. 2005-195998 (D2) discloses an arrangement, wherein an SMA wire is spirally wound around an outer circumference of a cam cylinder for guiding a lens element in an optical axis direction to pivotally move the cam cylinder by a contraction force of the SMA wire. Japanese Unexamined Patent Publication No. 2002-130114 (D3) discloses an arrangement, wherein an SMA wire is wound over a lens frame member and a fixed portion to move the lens frame member by a contraction force of the SMA wire. Also, Japanese Unexamined Patent Publication No. 2005-156892 (D4) discloses a lens driving mechanism including an arrangement of increasing a contraction force of an SMA wire by a gear mechanism.
Generally, a contraction displacement amount obtained by energizing and heating an SMA wire is no more than about several % with respect to the entire length of the SMA wire. Considering durability of the SMA actuator, it is necessary to contract the SMA actuator within a displacement range of not more than about 3%. In view of this, in the arrangements of directly driving a lens element i.e. a lens frame member by the SMA actuator, as disclosed in D1 through D3, the amount capable of moving the lens element is relatively small, and an intended lens moving amount required for high-performance auto-focus or optical zoom control cannot be obtained. Use of the arrangement of D4 may be advantageous in increasing a displacement amount. However, since the gear mechanism is mounted, the arrangement may hinder reduction of the lens driving mechanism in size and weight.